Effect of lacY Expression on Homogeneity of Induction from the Ptac and Ptrc Promoters by Natural and Synthetic Inducers

The role of the Escherichia coli lactose permease (LacY) in the homogeneous induction of the lactose-inducible promoters Ptac and Ptrc by the natural inducer lactose and the synthetic inducer isopropyl-beta-D-thiogalactopyranoside (IPTG) was investigated. Lactose requires active transport by LacY, whereas IPTG can freely penetrate the cell wall. In E. coli strains lacking a functional LacY, IPTG is required for induction of Ptac and Ptrc. In E. coli strains carrying a functional LacY, induction of Ptrc and Ptac with intermediate concentrations of lactose gave rise to two subpopulations, one fully induced and one uninduced, whereas a single, fully induced population resulted when high inducer concentrations were used. In contrast, induction with IPTG gave rise to a single population of cells at all inducer concentrations in both lacY and lacY^+ strains.